A gate valve has a body with a cavity intersected by a flow passage. A gate moves in the cavity between a closed position, blocking flow through the flow passage and an open position. A stem engages the gate, and when rotated, causes the movement of the gate. The stem extends through a stem passage in the body of the gate valve. Stem seals seal between the stem and the stem passage, preventing leakage of pressure from the cavity. The gate may be either a single slab or two split gates. The gate valve may be a non-rising type, wherein the stem has threads that engage mating threads in the gate. In the non-rising type, the stem does not move axially when rotated. Alternately, the gate valve may be a rising type wherein the stem has threads that engage threads in the valve body.
Gate valves often have an emergency feature, called a backseat seal, that when actuated, will block pressure in the cavity from the stem seals. The backseat seal normally comprises an annular rigid conical profile on the stem that is normally spaced below a mating conical seal surface at the junction of the cavity with the stem passage. During normal opening and closing operation, the profile will not contact the seal surface. To actuate, the operator rotates a threaded retainer or collar at the upper end of the stem passage to allow the stem to move upward past its normal operating position so that the profile seals against the seal surface.
While the backseat seal as described is a workable design, sometimes it does not seal well, particularly if the valve has to qualify for a high pressure rating. Also, the conical profile can become damaged during factory acceptance testing. If so, it is costly to replace the stem.